Ethernet – Identify gratuitous ARP

arpbroadcastethernetprotocol-theory

I want to identify a gratuitous ARP.

Can I say:
If sender IP address and target IP address equal, it's always a gratuitous ARP?

Best Answer

You can identify Gratuitous ARPs by looking at the ARP Sender Protocol Address and ARP Target Protocol Address, so as you mentioned when they are the same it's a gratuitous ARP. See RFC 2002, Section 4.6 for a reference (the emphasis - !!!!> and <!!!! is mine)...

      4.6. ARP, Proxy ARP, and Gratuitous ARP

      <...>

      A Gratuitous ARP [23] is an ARP packet sent by a node in order to
      spontaneously cause other nodes to update an entry in their ARP
      cache.  A gratuitous ARP MAY use either an ARP Request or an ARP
      Reply packet.  In either case, the (!!!!>) ARP Sender Protocol Address
      and ARP Target Protocol Address are both set to the IP address
      of the cache entry to be updated (<!!!!), and the ARP Sender Hardware
      Address is set to the link-layer address to which this cache
      entry should be updated.  When using an ARP Reply packet, the
      Target Hardware Address is also set to the link-layer address to
      which this cache entry should be updated (this field is not used
      in an ARP Request packet).

      In either case, for a gratuitous ARP, the ARP packet MUST be
      transmitted as a local broadcast packet on the local link.  As
      specified in [16], any node receiving any ARP packet (Request or
      Reply) MUST update its local ARP cache with the Sender Protocol
      and Hardware Addresses in the ARP packet, if the receiving node
      has an entry for that IP address already in its ARP cache.  This
      requirement in the ARP protocol applies even for ARP Request
      packets, and for ARP Reply packets that do not match any ARP
      Request transmitted by the receiving node [16].

Related Solutions

ARP request outside of LAN; Target machine or router response

Using your original diagram:

OP's original diagram

When Computer A tries to communicate with Computer C, the following steps resolve Router 1's software-assigned address to its hardware-assigned media access control address: Based on the contents of the routing table on Computer A, IP determines that the forwarding IP address to be used to reach Computer C is through Router 1, the IP address of its default gateway. Host A then checks its own local ARP cache for a matching hardware address for Router 1.

If Computer A finds no mapping in the cache, it broadcasts an ARP request frame to all hosts on the local network with the question "What is the hardware address for Router 1?" Both hardware and software addresses for the source, Host A, are included in the ARP request.

Each host on the local network receives the ARP request and checks for a match to its own IP address. If a host does not find a match, it discards the ARP request.

Router 1 determines that the IP address in the ARP request matches its own IP address and adds a hardware/software address mapping for Host A to its local ARP cache.

Router 1 then sends an ARP reply message containing its hardware address directly back to Host A.

When Host A receives the ARP reply message from the router, it updates its ARP cache with a hardware/software address mapping for Router 1.

Once the media access control address for Router interface 1 has been determined, Host A can send IP traffic to Router 1 by addressing it to the Router interface 1 media access control address. The router then forwards the traffic to Host C through the same ARP process as discussed in this section.

This was updated from a Microsoft Technet article to match your example. Another reference with a good example is Juniper networks description.

In a nutshell Host A when communicating with external IP addresses/hosts will look to its default gateway for external IP resolution and assume traffic to Host C will be forwarded by this gateway.

ARP Protocol – Understanding Packet Types, Ethernet, Cache, and Gratuitous ARP

When in doubt, go to the source, RFC 826, An Ethernet Address Resolution Protocol

There are two type of ARP messages: REQUEST and REPLY.
The RFC specifies the ethernet header. An ARP request involves determining the ethernet address of the destination, so it uses the broadcast address since it doesn't know the unicast address. An ARP reply will know the ethernet address, so it will be a unicast.
```
Ethernet transmission layer (not necessarily accessible to the user):  
    48.bit: Ethernet address of destination  
    48.bit: Ethernet address of sender  
    16.bit: Protocol type = ether_type$ADDRESS_RESOLUTION
```
It then causes this packet to be broadcast to all stations on the Ethernet cable originally determined by the routing mechanism.
The way the ARP cache works is OS-dependent, and there is nothing requiring a host to maintain an ARP cache.
The use of gratuitous ARP is not officially documented, but some hosts and OSes use it to do things like resolve address conflicts; Wireshark has a pretty good explanation (with examples): Gratuitous ARP

Gratuitous ARP could mean both gratuitous ARP request or gratuitous ARP reply. Gratuitous in this case means a request/reply that is not normally needed according to the ARP specification (RFC 826) but could be used in some cases. A gratuitous ARP request is an AddressResolutionProtocol request packet where the source and destination IP are both set to the IP of the machine issuing the packet and the destination MAC is the broadcast address ff:ff:ff:ff:ff:ff. Ordinarily, no reply packet will occur. A gratuitous ARP reply is a reply to which no request has been made.

Best Answer

Related Solutions

ARP request outside of LAN; Target machine or router response

ARP Protocol – Understanding Packet Types, Ethernet, Cache, and Gratuitous ARP

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